0
0
+45
-45
0
90
90
0
-45
+45
0
0
0
0
+45
-45
0
90
90
0
-45
+45
0
0
Pyramid pattern matrix crack from impact
Low Energy Impact
0
0
+45
-45
0
90
90
0
-45
+45
0
0
0
0
+45
-45
0
90
90
0
-45
+45
0
0
Delaminations Back side fiber fracture
Local fiber/matrix crushing
Medium Energy Impact
0
0
+45
-45
0
90
90
0
-45
+45
0
0
0
0
+45
-45
0
90
90
0
-45
+45
0
0
Delaminations
High Energy Impact
Through
penetration
small damage
zone
Loose
fiber ends
Figure 3-16. Impact energy affects the visibility, as well as the
severity, of damage in composite structures. High and medium
energy impacts, while severe, are easy to detect. Low energy impacts
can easily cause hidden damage.
medium and high energy impacts, the damage is visible to the
eye, but low energy impact is difficult to detect. [Figure 3-16]
If an impact results in delaminations, crushing of the surface,
or a puncture, then a repair is mandatory. While waiting
for the repair, the damaged area should be covered and
protected from rain. Many composite parts are composed
of thin skins over a honeycomb core, creating a “sandwich”
structure. While excellent for structural stiffness reasons,
such a structure is an easy target for water ingress (entering),
leading to further problems later. A piece of “speed tape”
over the puncture is a good way to protect it from water, but
it is not a structural repair. The use of a paste filler to cover
up the damage, while acceptable for cosmetic purposes, is
not a structural repair, either.
The potential for heat damage to the resin is another
disadvantage of using composites. While “too hot” depends
on the particular resin system chosen, many epoxies begin
to weaken over 150 °F. White paint on composites is often
used to minimize this issue. For example, the bottom of
a wing that is painted black facing a black asphalt ramp
on a hot, sunny day can get as hot as 220 °F. The same
structure, painted white, rarely exceeds 140 °F. As a result,
composite aircraft often have specific recommendations
on allowable paint colors. If the aircraft is repainted, these
recommendations must be followed. Heat damage can also
occur due to a fire. Even a quickly extinguished small brake
fire can damage bottom wing skins, composite landing gear
legs, or wheel pants.
Also, chemical paint strippers are very harmful to composites
and must not be used on them. If paint needs to be removed
from composites, only mechanical methods are allowed, such
as gentle grit blasting or sanding. Many expensive composite
parts have been ruined by the use of paint stripper and such
damage is generally not repairable.
Fluid Spills on Composites
Some owners are concerned about fuel, oil, or hydraulic fluid
spills on composite surfaces. These are generally not a problem
with modern composites using epoxy resin. Usually, if the
spill does not attack the paint, it will not hurt the underlying
composite. Some aircraft use fiberglass fuel tanks, for example,
in which the fuel rides directly against the composite surface
with no sealant being used. If the fiberglass structure is made
with some of the more inexpensive types of polyester resin,
there can be a problem when using auto gas with ethanol
blended into the mixture. The more expensive types of
polyester resin, as well as epoxy resin, can be used with auto
gas, as well as 100 octane aviation gas (avgas) and jet fuel.
Lightning Strike Protection
Lightning strike protection is an important consideration in
aircraft design. When an aircraft is hit by lightning, a very
large amount of energy is delivered to the structure. Whether
flying a light general aviation (GA) aircraft or a large airliner,
the basic principle of lightning strike protection is the same.
For any size aircraft, the energy from the strike must be spread
over a large surface area to lower the amps per square inch
to a harmless level.
If lightning strikes an aluminum airplane, the electrical
energy naturally conducts easily through the aluminum
structure. The challenge is to keep the energy out of avionics,
fuel systems, etc., until it can be safely conducted overboard.
The outer skin of the aircraft is the path of least resistance.
Figure 3-17. Composite materials in aircraft, such as Columbia 350
(top), Boeing 787 (middle), and a Coast Guard HH-65 (bottom).
In a composite aircraft, fiberglass is an excellent electrical
insulator, while carbon fiber conducts electricity, but not
as easily as aluminum. Therefore, additional electrical
conductivity needs to be added to the outside layer of
composite skin. This is done typically with fine metal meshes
bonded to the skin surfaces. Aluminum and copper mesh
are the two most common types, with aluminum used on
fiberglass and copper on carbon fiber. Any structural repairs
on lightning-strike protected areas must also include the mesh
as well as the underlying structure.
For composite aircraft with internal radio antennas, there
must be “windows” in the lightning strike mesh in the area
of the antenna. Internal radio antennas may be found in
fiberglass composites because fiberglass is transparent to
radio frequencies, where carbon fiber is not.
The Future of Composites
In the decades since World War II, composites have earned
an important role in aircraft structure design. Their design
flexibility and corrosion resistance, as well as the high
strength-to-weight ratios possible, will undoubtedly continue
to lead to more innovative aircraft designs in the future.
From the Cirrus SR-20 to the Boeing 787, it is obvious that
composites have found a home in aircraft construction and
are here to stay. [Figure 3-17]
Instrumentation: Moving into the Future
Until recently, most GA aircraft were equipped with
individual instruments utilized collectively to safely operate
and maneuver the aircraft. With the release of the electronic
flight display (EFD) system, conventional instruments have
been replaced by multiple liquid crystal display (LCD)
screens. The first screen is installed in front of the pilot
position and is referred to as the primary flight display (PFD).
The second screen, positioned approximately in the center
of the instrument panel, is referred to as the multi-function
display (MFD). These two screens de-clutter instrument
panels while increasing safety. This has been accomplished
through the utilization of solid state instruments that have
a failure rate far less than those of conventional analog
instrumentation. [Figure 3-18]
With today’s improvements in avionics and the introduction
of EFDs, pilots at any level of experience need an astute
knowledge of the onboard flight control systems, as well as
an understanding of how automation melds with aeronautical
decision-making (ADM). These subjects are covered in detail
in Chapter 2, Aeronautical Decision-Making.
Whether an aircraft has analog or digital (glass) instruments,
the instrumentation falls into three different categories:
performance, control, and navigation.
Performance Instruments
The performance instruments indicate the aircraft’s actual
performance. Performance is determined by reference to the
altimeter, airspeed or vertical speed indicator (VSI), heading
indicator, and turn-and-slip indicator. The performance
instruments directly reflect the performance the aircraft
is achieving. The speed of the aircraft can be referenced
on the airspeed indicator. The altitude can be referenced
on the altimeter. The aircraft’s climb performance can be
determined by referencing the VSI. Other performance
instruments available are the heading indicator, angle of
attack indicator, and the slip-skid indicator. [Figure 3-19]
Figure 3-18. Analog display (top) and digital display (bottom) from
a Cessna 172.
Control Instruments
The control instruments display immediate attitude and power
changes and are calibrated to permit adjustments in precise
increments. [Figure 3-20] The instrument for attitude display
is the attitude indicator. The control instruments do not
indicate aircraft speed or altitude. In order to determine these
variables and others, a pilot must reference the performance
instruments.
Navigation Instruments
The navigation instruments indicate the position of the
aircraft in relation to a selected navigation facility or fix.
This group of instruments includes various types of course
indicators, range indicators, glideslope indicators, and
bearing pointers. Newer aircraft with more technologically
advanced instrumentation provide blended information,
giving the pilot more accurate positional information.
Navigation instruments are comprised of indicators that
display GPS, very high frequency (VHF) omni-directional
radio range (VOR), nondirectional beacon (NDB),
and instrument landing system (ILS) information. The
instruments indicate the position of the aircraft relative to a
selected navigation facility or fix. They also provide pilotage
information so the aircraft can be maneuvered to keep it on
a predetermined path. The pilotage information can be in
either two or three dimensions relative to the ground-based or
space-based navigation information. [Figures 3-21 and 3-22]
Global Positioning System (GPS)
GPS is a satellite-based navigation system composed of a
network of satellites placed into orbit by the United States
Department of Defense (DOD). GPS was originally intended
for military applications, but in the 1980s the government
made the system available for civilian use. GPS works in
all weather conditions, anywhere in the world, 24 hours a
day. A GPS receiver must be locked onto the signal of at
least three satellites to calculate a two-dimensional position
(latitude and longitude) and track movement. With four or
more satellites in view, the receiver can determine the user’s
three-dimensional position (latitude, longitude, and altitude).
Other satellites must also be in view to offset signal loss
and signal ambiguity. The use of the GPS is discussed in
more detail in Chapter 16, Navigation. Additionally, GPS is
discussed in the Aeronautical Information Manual (AIM).
Chapter Summary
This chapter provides an overview of aircr aft structures.
A more in-depth understanding of aircraft structures and
controls can be gained through the use of flight simulation
software or interactive programs available online through
aviation organizations, such as the Aircraft Owners and Pilots
Association (AOPA). Pilots are also encouraged to subscribe
to or review the various aviation periodicals that contain
valuable flying information. As discussed in Chapter 1, the
National Aeronautics and Space Administration (NASA) and
the FAA also offer free information for pilots.